Field of the Invention
The present disclosure relates to a fixing apparatus for fixing a toner image onto a recording medium and relates to a heater for use in the apparatus.
Description of the Related Art
Image forming apparatuses, such as electrophotographic copying machines and printers, are equipped with a fixing apparatus. Japanese Patent Laid-Open No. 08-234598 discloses a ceramic heater including a heat generation resistor disposed on a ceramic substrate, feeding electrodes for supplying electric power to the heat generation resistor, and an overcoat layer disposed so as to coat the heat generation resistor.
With this fixing apparatus, energization of the heat generation resistor is controlled so that the ceramic heater is heated, and the ceramic heater is pushed against a pressure roller with a heat-resistive fixing film in between. A recording medium on which an unfixed toner image is formed passes between the fixing film and the pressure roller, so that the toner image is fixed on the recording medium. In such a fixing apparatus, an energization control unit that controls energization of the heat generation resistor can fail to operate properly (cannot control energization). In this case, abnormal heat generation of the ceramic heater has to be prevented.
FIG. 14 illustrates a power feeding circuit for a heater 1301. In FIG. 14, a current suppression device 1305 having positive temperature coefficient (PTC) properties, a protection device 1309, such as a thermistor, an energization control device 1401, such as a relay, and an alternate-current source are connected in series to the heater 1301. The energization control device 1401 is controlled by a CPU 1402 on the basis of the detection result of a temperature sensor 1310 that detects the temperature of the heater 1301.
When the energization control device 1401 is damaged due to short-circuit, the heater 1301 can excessively rise in temperature and be broken due to thermal stress. Although the protection device 1309 is provided for an excessive rise in the temperature of the heater 1301, the heater 1301 can be broken before the protection device 1309 operates owing to a delay in response of the protection device 1309. However, with the configuration of FIG. 14, the resistance of the current suppression device 1305 increases when the current suppression device 1305 is heated. This reduces the amount of current flowing through a heat generation resistor of the heater 1301 even if the energization control device 1401 is damaged due to shorts-circuit, preventing the heat generation resistor from overheating. This decreases the rate of temperature rise of the heater 1301 compared with a case without the current suppression device 1305, preventing the heater 1301 from being broken before the protection device 1309 operates.
However, in a case in which a current suppression device having a positive temperature coefficient property is used in a fixing apparatus that uses a ceramic heater, the current suppression device has to be connected in series to the heater and to dispose the current suppression device in the vicinity of the heater. Furthermore, with the size reduction of image forming apparatuses, it has become difficult to dispose a reinforced insulation structure defined by a safety standard, such as IEC60950, between a power supply to a ceramic heater and the ground. For this reason, a protection device (for example, a thermal cutoff) adhering to the standard has to be connected in series to the ceramic heater.
One example of a position at which the current suppression device can easily receive the heat of the heater is the back of a heater holder (the opposite surface of the heater holder from the surface that holds the heater). However, in addition to the current suppression device, a protection device and a temperature sensor have to be disposed on the back of the heater holder. For this reason, the configuration in which the current suppression device is disposed on the back of the heater holder hinders reduction in the size of the product.